Solid-state light-emitting elements, such as LED elements, are expected to become light sources for various products due to their small size, high efficiency, and long life span.
The voltage-current characteristics of an LED element have a non-linear feature in which an electric current (hereinafter, simply referred to as current) starts flowing at a certain applied voltage or higher, and a forward voltage does not substantially change while a current near a rated current value is flowing. Hence, a light output of the LED element basically depends on a value of a current flowing through the LED element.
In illumination uses, an LED unit 1, illustrated in FIG. 1 and including a plurality of LED elements 2 connected in series-parallel, is used as a light source, so as to obtain a light output of a predetermined luminance.
As described above, the light output of the LED element depends on a value of a current flowing through the LED element. Hence, the current value corresponding to the light output of a predetermined luminance is set as a rated current value of the LED unit 1.
Accordingly, the lighting device that lights up the LED unit 1 is desirably controlled such that a constant current is supplied to the LED unit 1.
FIG. 2 is a circuit diagram of an example of a lighting device for an LED unit. FIG. 2 illustrates not only the lighting device, but also a DC power source E1 that supplies a DC power to the lighting device, and an LED unit 23 connected to the lighting device.
The lighting device includes a step-down chopper circuit 21 that is one type of a DC/DC converter, and a control circuit 22. The step-down chopper circuit 21 includes a switching element Q1, an inductor L1, a diode D1, an output capacitor C1, and the like.
The step-down chopper circuit 21 further includes a current detection circuit IS1 and a diode voltage detection circuit VS1. The current detection circuit IS1 detects a current value Isense1 flowing through the inductor L1. The diode voltage detection circuit VS1 detects a voltage value Vsense1 across the diode D1.
Examples of the DC power source E1 include a DC power source that includes a commercial AC power source and a full-wave rectifier circuit, and a DC power source that includes a commercial AC power source and a power factor improvement circuit.
Now, a brief description is given of an operation of the lighting device illustrated in FIG. 2.
When the switching element Q1 is turned ON in response to a command from the control circuit 22, a current flows through the LED unit 23 from the DC power source E1 via the switching element Q1, the inductor L1, and the output capacitor C1. The current flowing through the inductor L1 has a time rate of change of (Vin−Vout)/L that is determined by a voltage value Vin of the DC power source E1, a load voltage Vout applied to the LED unit 23, and an inductance value L of the inductor L1.
The switching element Q1 is turned OFF when the above current is detected by the current detection circuit IS1 and a detected value Isense1 reaches a target current value Iref.
When the switching element Q1 is turned OFF, energy stored in the inductor L1 is released with a current supplied while the switching element Q1 was ON.
While the energy stored in the inductor L1 is being released, the diode D1 is conducting. At this time, the diode has a forward voltage that has a very small value. When the diode D1 becomes non-conducting after the energy release, the voltage across the diode D1 rises to a value near the load voltage Vout.
The rise of the voltage across the diode D1 is detected through detection that a voltage value Vsense1 that is an output of the diode voltage detection circuit VS1 has exceeded a predetermined value Vref. When the rise of the voltage across the diode D1 is detected, the control circuit 22 determines that the release of the energy stored in the inductor L1 is finished, and turns ON the switching element Q1 again.
FIG. 3 is a circuit diagram illustrating an example of the control circuit 22 that operates as described above.
The control circuit 22 includes comparators 31 and 32, and an RS flip-flop.
The comparator 31 is an element that detects a current flowing through the inductor L1. The comparator 32 is a circuit that detects a voltage generated in the diode D1.
The RS flip-flop is a circuit that receives I_detect that is an output of the comparator 31 as a reset signal, and receives V_detect that is an output at a connection point 35 of the comparator 32 as a set signal. The RS flip-flop that outputs a signal to a connection point 36 is formed by NOR circuits 33 and 34 illustrated in FIG. 3.
FIG. 4 is a timing chart illustrating operations of respective elements in the lighting device described above.
In FIG. 4, currents flowing through the switching element Q1, the diode D1, and the inductor L1 are respectively represented by I_Q1, I_D1, and I_L1.
By use of the lighting device that operates as described above, energy is continuously supplied to the lighting device, and a stable DC current is supplied from the output capacitor C1 to the LED unit 23 serving as a load. In addition, in principle, an appropriate setting of a current peak value of the inductor L1 allows the LED unit 23 to be lit up with a rated current.
Moreover, there are conventional techniques for enhancing stabilization of a current flowing through an LED unit. Japanese Unexamined Patent Application Publication No. 2012-109141 (hereinafter, referred to as patent literature (PTL) 1) discloses a method below performed in a lighting device including a step-down chopper circuit. Specifically, the method is performed to keep a current flowing through an LED unit constant regardless of the variations in DC power source when an energy release of an inductor is detected by a voltage generated at a secondary winding of the inductor. In the technique disclosed in PTL 1, when a switching element is OFF, the voltage generated at the secondary winding of the inductor is used to keep a current flowing through an LED unit serving as a load constant even when a DC power source varies. In this way, the variations in current due to a power source voltage is reduced.